Mobile phone WiFi calling function blocked on WiFi unless the phone is with O2

Hi all,

Here is what I believe to be a very difficult one but maybe you'll prove me wrong. We have various different WiFi networks being broadcast around our organisation e.g Guest, Corporate.

When connected to any of the WiFi networks, the WiFi calling feature on the mobiles fails to kick in unless that phones cellular provider is O2 (UK cellular provider)

I have checked our firewall and can see no reason for this function to be blocked, especially seeing as the traffic would be initiated from inside to out and therefore permitted back in due to packet inspection.

We also run firepower and I have also ruled that out as the cause for the function not working.

The external IP these networks are routed outbound on is a Houston IP address however the internal subnets are based in the UK. Could this function be failing to work for all UK cellular providers bar one due to the external IP address being US based ?

Thanks in advance